ZFS RaidZ2 with 24 drives?

[Šimun Mikecin]

1. sij. 2010., u 20:51, Bernd Walter <ticso at cicely7.cicely.de> napisao:

On Fri, Jan 01, 2010 at 10:56:21AM -0600, Bob Friesenhahn wrote:
On Fri, 1 Jan 2010, Danny Carroll wrote:

You do not have this protection when ZFS has access to the raw devices.
Even worse if the devices write cache is turned on.

This statement does not appear to be true.  ZFS will always request 
that devices flush their cache.  The only time there is no 
"protection" is if the device ignores that flush request and the cache 
is volatile.  Controller battery-backed RAM is useful since the 
controller can respond to the cache flush request once the data is in 
battery-backed RAM, thereby dramatically improving write latencies for 
small writes

Which - if it is true for the controller - can be dangerous.
A battery backed cache is volatile if the system is going down for
a long time.
Or consider the system is going down to relocate the disks to a new
machine, or just to a newer controller?


If you are using amr driver then FreeBSD will flush cache during shutdown. Haven't tried other drivers myself, but I suppose they also do the same.
You can have a problem only on unclean shutdown after which you didn't reboot (nobody does that willingly).